An extended-family study of caries susceptibility among the demes of a discrete, isolated population is proposed. We will visit twelve colonies of the Hutterite sect, an Anabaptist group which numbers approximately 20,000 individuals. The population constitutes a literate, non-mobile group in which all generations are represented locally, which does not practice birth control, and which has exhaustive geneological and vital statistics records, uniform living conditions, no in-migration and virtually no illegitimacy. Pilot data obtained at three colonies have confirmed the existence of a wide range of caries experience among these people, strong familial effects and, most importantly, large differences in DMFT scores among sibs of comparable age. The effects of diet, natural fluoridation, and oral hygiene will be rigorously assessed, and the range of population variation in carious lesions will be determined between and within pedigrees. Genetic and environmental contributions to the variance will be estimated by path analysis, and major gene effects will be investigated by pedigree analysis. Thus the probable mode of inheritance as well as the magnitude of the effect of any host factors will be specifiable. The proposed research constitutes a unique opportunity to bring strict genetic control to the study of caries prevalence cross-sectionally. Positive results will provide a solid basis for a longitudinal study of the nature of any host factors and of the rate of disease process in this same population.